This invention relates to the generation of hydrogen, and more particularly this invention relates to a method and apparatus for catalytic hydrogenation and of the evolution of hydrogen.
The decade of the 1980's has become a critical period for mankind, energy consumption has continued to increase throughout the world as a result of the population explosion, accelerated industrialization, economic growth, and social development.
There is an increased public awareness of the social and environmental problems related to the production and consumption of fuels.
Efficiency in the production of energy, is one of the more critically important technical problems of the day. At the very time that the world's economy, and the economies of the industralized countries are becoming increasingly dependent on the consumption of energy, there is a growing realization, that the main sources of this energy, the earth's non-renewable fossil fuel reserves, will inevitably be exhausted, and that in any event, the natural environment of the earth cannot readily assimilate the by-products of fossil fuel at much higher rates than it does at present without suffering unacceptable levels of pollution.
There is considerable agreement among scientists around the world, that hydrogen as fuel will emerge as the ultimate clean energy for the future. One of the raw materials out of which hydrogen is derived, and covers 71% of the total surface area of the earth, is water. Hydrogen is a non-pollutant fuel, and when burnt with air, it produces heat and steam, and as we all know, when it is condensed, water. This process is similar to the kind of recycling that nature provides, therefore, it works in harmony with nature, not in opposition to it.
There are many known methods to produce hydrogen, unfortunately they are expensive. Another problem standing in the way of hydrogen as a fuel, is the difficulty to store it, whether on board a vehicle or in a suitable stationary application, either as a compressed gas, or as a cryogenic liquid. Storage of hydrogen as a hydride appears promising, but it has an inherent high weight penalty associated with it.
It follows, that more efficient energy conversion processes and equipment must be provided, also, at any cost an alternate fuel supply for all of planet earth must be developed; one that will not deplete our natural resources in the manner of today.
It is known that the palladium/hydrogen system has been perhaps the most extensively experimentally investigated metal/gas system. It represents a general problem in inorganic binary systems concerning departures of the composition of the solid phase from simple stoichiometric ratios dependent on temperature and the equilibrium pressure of the gaseous component.
The system has further specific interest in that even with high contents of hydrogen, the solid phase retains considerable mechanical robustness in addition to its retention of metallic appearance and the considerable electrical conductivity also broadly characteristic of other hydrides of the transition elements. From an academic standpoint, interest in the system has perhaps centred primarily around attempts to provide theoretical explanations both of the form in which the hydrogen is contained in the solid and of the form of the pressure composition relationships.
Combinations of practical and theoretical interests have arisen from the applications of palladium and certain palladium alloys as hydrogen diffusion membranes, which also partly stems from a rather unique resistance to disruption and serious embrittlement even after absorbing large concentrations of hydrogen. These features of the system can, furthermore, also prove of practical advantage in the role of palladium (as well as of allied elements; for example, of the platinum group) in the fields of catalytic hydrogenation and of the evolution of hydrogen.
Using permeable palladium membranes for hydrogen production from one of the earth's most abundant resources, water, in a process now under study, by Johnson Space Center Code AT3, Houston, Tex. 77058, U.S.A. whereby superheated steam is fed into a long tunnel made of a permeable membrane. This input superheated steam consists of a natural balance, or equilibrium, between water vapour and a small percentage of free hydrogen and oxygen. As the steam moves along the tunnel, the initial free hydrogen escapes through the membrane, and the water vapour and the oxygen remain inside the tunnel. This upsets the natural water/oxygen/hydrogen equilibrium in the steam, and results in the decomposition of some of the water molecules into more hydrogen and oxygen. Some of this hydrogen is removed thereby causing further decomposition of water molecules to provide more hydrogen for subsequent removal. Thus, as the gaseous mixture passes through the tunnel, hydrogen will be continuously formed and removed.
In the past, attempts had been made to remove free hydrogen from water using selectively permeable membranes. The productivity of these methods was limited to the recovery of the small amounts of free hydrogen naturally present in the water vapour. The features of this new process are:
a. The flowing vapour is contained within the membrane, under conditions favouring decomposition, for a long period of time. PA0 b. This extended period of time allows the water (oxygen/hydrogen) system to produce hydrogen through the decomposition of water. PA0 c. The process continues, and new hydrogen is produced as long as the gaseous mixture is in the tunnel. PA0 1. All the substances composing the system. PA0 2. The quantity and physical state--Gas, liquid, solid and when appropriate, the crystalline form of each of these substances. PA0 3. The temperature and pressure of the system. PA0 Enthalpy of Fusion (.DELTA.H.sub.fus) PA0 Vapourisation (.DELTA.H.sub.vap) and PA0 Sublimation (.DELTA.H.sub.subl)
However, even this new process or method of producing hydrogen needs to be improved by increasing its production efficiency, which in turn will improve its economic potential.
Before proceeding with the detailed description of the method and apparatus of the present invention, a detailed discussion of the principles upon which known methods of producing hydrogen from water or steam in addition to the process described above is in order.
At ordinary temperatures hydrogen can be displaced from water by the action of highly electropositive metals, or by electrolysis. Hydrogen can also be produced by the reaction of the hydrides of highly electropositive metals such as LiH, and CaH.sub.2, using water at ordinary temperatures.
At higher temperatures, hydrogen may be displaced from water by a few of the less electropositive metals, and some of the non-metals. Although some of these reactions are convenient for laboratory use, only one or two have been used commercially on a large scale.
The metals which can be used at higher temperatures, i.e. magnesium, manganese, zinc and iron, do not displace hydrogen from water at room temperature, but do so if the metals are heated and the water is in the form of steam. The reaction of these metals with steam at high temperature produces hydrogen gas and the oxide rather than the hydroxide of the metal.
For example, when steam is passed over heated magnesium, the magnesium burns brightly producing solid magnesium oxide and hydrogen gas: EQU (heated)Mg+H.sub.2 O(steam).fwdarw.H.sub.2 +MgO
and, .DELTA.H=-86.0 K cal/mole equation. Zinc, iron, and manganese also react with steam when heated, though less readily than magnesium, yielding hydrogen gas and zinc oxide, iron oxide, and manganese oxide respectively.
All of the above reaction are exothermic, so that once they are started they generally give off enough heat to maintain or supplement to great extent the high temperatures required for the reaction to maintain itself at the required rate.
Certain non-metals, such as carbon, can also displace hydrogen from water at high temperature, producing an oxide of the non-metal and hydrogen gas: EQU (heated)C+H.sub.2 O(steam).fwdarw.H.sub.2 +CO
and, .DELTA.H=+31.4 K cal/mole equation this reaction is endothermic, so it is necessary to supply heat to maintain the reaction.
The Bosch Process:
Steam is passed over incandescent coke, at about 1,000.degree. C. to produce a mixture of hydrogen and carbon monoxide called water gas. This is an endothermic reaction and a continuing supply of coke is consumed to maintain the temperatures essential to the reaction.
The hot carbon acts as a reducing agent and removes the oxygen from the steam. Since heat is absorbed in the reaction, the temperature of the coke drops as the reaction proceeds, and consequently the reaction slows down and would eventually stop. The coke therefore, must be reheated if continuous formation of water gas is required.
In actual practice, this is accomplished by alternating the endothermic stage, the production of water gas, with the exothermic stage of reheating the coke. During the reheating of the coke it is being continuously consumed, and of course the residue must be removed and replaced with fresh coke and heated. Adequate heat is evolved in the burning of the coke to raise the temperature to a red incandescent heat, which can then react further with the steam to produce the mixture of hydrogen and carbon monoxide.
A variation of the Bosch Process uses petroleum hydrocarbons instead of coke. Reforming reactions between hydrocarbons and steam in the presence of suitable catalysts at high temperatures (1,100.degree. C.) produce a mixture of H.sub.2 and CO. The hydrocarbon methane reacts with steam: EQU (gas)CH.sub.4 +H.sub.2 O(steam) and (cat).fwdarw.3H.sub.2 +CO
and .DELTA.H=+49.3 K cal/mol equation. This mixture of H.sub.2 gas and CO gas may be used as a fuel. However, if hydrogen is the desired product, then the two gases must be separated by further reacting the carbon monoxide gas with additional steam at a lower temperature (500.degree. C.), in the presence of a catalyst to produce more hydrogen gas and carbon dioxide gas. These two can be separated by passing the mixture through cold water under high pressure. The carbon dioxide gas dissolves, and the hydrogen gas is collected over the water.
Each and every one of the known methods of producing hydrogen, or a fuel gas which contains hydrogen, share a common fault or weakness. Each and every one of these methods require a second consumable, other than water, in order to produce hydrogen. The practicality and availability of this second consumable is always prohibitive for commerical acceptance, due to the ever increasing cost of raw materials used for this type of energy production.
Additionally, the methods which could possibly be operated on a large scale are all cyclic in nature, therefore, they cannot be operated continuously, which makes them ineffective as a system to produce hydrogen on board a vehicle, as well as in any other application where H.sub.2 is required in a continuous manner.
Accordingly, it is the object of the present invention to provide a method of producing hydrogen from water which is in the form of steam by catalytic reaction.
It is another object of the present invention to provide a method for producing hydrogen from steam wherein the only consumable is water.
It is still another object of the present invention to provide a method for the production of hydrogen from steam which is highly efficient.
It is yet another object of the present invention to provide a method for generating hydrogen from steam in a continuous manner.
It is still another object of the present invention to provide an apparatus for generating hydrogen from steam.
It is yet another object of the present invention to provide a method and apparatus for generating hydrogen from steam, using permeable diffusion membranes, which assists the decomposition of steam and provide a means to displace hydrogen in a continuous manner, as well as using an additional metal catalyst capable of absorbing oxygen, therefore producing hydrogen, and capable of releasing oxygen at a predetermined dissociation temperature.
It is still a further object of the present invention to provide an apparatus for generating hydrogen from steam on board a vehicle as well as, in any stationary application where hydrogen is required on demand.
Because the present invention is so involved with the basic Laws of Science, they will be referred to individually as they become relevant. When appropriate or meaningful, I will not only specify the thermodynamic law involved, but I will insert equations as related to the discussion of the moment.
It is known that there are many kinds of energy, such as heat energy, radiation, mechanical, electrical, and magnetic energy, and so on.
Furthermore, from the Law of Conservation of Energy, it is known that the type of physical and chemical processes discussed in relation to the invention agrees, that in a chemical change there is no loss or gain but merely a transformation of energy from one form to another. The Conservation of Energy Law states, that "energy can neither be created nor destroyed" and therefore the total amount of energy in the universe remains constant.
This statement does not contradict the principle of the invention, because the law merely implies that whenever heat is actually converted to another form of energy, an equivalent amount of the other form appears.
When reference is given to a thermodynamic system, I intend that, it comprise any substance or substances under consideration. For example, 1 mole of a given gas under specified conditions of volume, pressure, and temperature can be considered a system. A given quantity of water at a given temperature and pressure may be a system, and so may a known quantity of solid iron together with a known quantity of oxygen gas at a given temperature and pressure.
The surroundings consist of everything outside the system. Specifically, the environmental surrounding, which is somehow effected by the change occurring in the system. A closed system does not exchange matter with its surroundings, but may exchange energy in some form--heat for example.
An isolated system exchanges neither matter nor energy with its surroundings.
When reference is given to a system as being in a given state, the system is fully characterized, and all relevant conditions are specified. For example, if the system is an ideal gas, its state is determined by any three of the following four properties:
Temperature--T PA1 Pressure--P PA1 Volume--V PA1 Number of Moles--N PA1 H=Enthalpy or heat content PA1 E=Internal energy of the system PA1 P=Pressure PA1 V=Volume
One can say any three because the fourth property may be derived from the ideal gas law, PV=nRT, or from the Van der Waals equation, (P+a/V.sup.2)(V-b)=RT for one mole of gas.
For a system composed of a specified number of moles of a pure liquid or a pure solid, the temperature and pressure are sufficient to completely describe the state of a stable system i.e., at equilibrium. Therefore, if one considers 1 mole of liquid water at 20.degree. C. and under total pressure of 1 ATM, we can find from tables of physical constants that the density is 1.0 G/CM.sup.3, and consequently we know that 1 mole (18.06 G) of liquid H.sub.2 O occupies a volume of 18.0 CM.sup.3.
For solids, there is the possibility that different crystalline modifications may exist indefinitely, even though only one form is stable. In this case the crystalline form must be specified.
Finally, when the system is a mixture of two or more substances in equilibrium, along with temperature and pressure, the quantity of each substance present will be specified.
To summarise; the state of a thermodynamic system in the following description will be indentifiable by specifying:
Transformation of a system, indicates any process by which a system passes from a specified initial state to a specified final state. Any chemical reaction represents a transformation of a system; the initial state of the system comprises the reactants, and the final state comprises the products.
Enthalpy and its relation to the invention:
Enthalpy, or heat content, is a thermodynamic quantity. It is equal to the sum of the internal energy of a system plus the product of pressure-volume work done on the system. EQU thus H=E+PV
where
A transformation at a constant pressure P, in which no work is done except the pressure-volume work, is w=-P.DELTA.V. For such transformations, the first law, .DELTA.E=Q+W can be written as: EQU .DELTA.E=Q-P.DELTA.V
where P=constant. Since P is always a positive quantity, the term P.DELTA.V has a negative sign if the work is done by the surroundings on the system, and it has a positive sign if the work is done by the system on the surroundings. The energy crossing the system boundary under the influence of a temperature difference or gradient is a quantity of heat Q represents an amount of energy in transit between the system and its surroundings and is not a property of the system. The usual convention with respect to signs requires that numerical values of Q be taken as positive when heat is added to the system, and negative when heat leaves the system. This is then called the enthalpy change of the transformation, .DELTA.H. Therefore, for any transformation at constant pressure and involving only pressure-volume work, the first law is written as: EQU .DELTA.E=.DELTA.H-P.DELTA.V and .DELTA.H=.DELTA.E+P.DELTA.V
The quantitative enthalpy relationships with which thermochemistry is concerned are based on the Law of Conservation of Energy, and are valid for any given chemical change under consideration, regardless of structural interpretation of the chemical change itself. Thermochemical data has been used, to assist in determining which are the more significant energy factors relating to our development and to check the validity of our extrapolations of them.
During a reaction, the temperature of the system may rise and fall, and so may the pressure, but these changes will not effect the values of .DELTA.H, which is the enthalpy change of the reaction when the final state of the system, the products, has returned to the temperature and pressure of the initial state of the system. It is noted then, the enthalpy of a system is a thermodynamic function (a function of state), so that the enthalpy of a system accompanying a reaction, .DELTA.H=H.sub.f -H.sub.i, is independant of any intermediate state or states.
When it is not relevant to the equation, the physical state, temperature, and pressure of each of the reactants and products of a thermochemical reaction will be expressed as standard state. Then, any enthalpy change involving substances in their standard states is called standard enthalpy change at the specified temperature, and is indicated as .DELTA.H.
The initial step in the solution of our problem in thermodynamics applied to this invention, once the problem is established is to translate it into the terminology of thermodynamic variables, so that the laws of thermodynamics may be imposed. Therefore, one must provide the terms used to describe the quantitative definiations of the enthalpy (heat) changes which accompany chemical reactions.
Enthalpy of Atomization (.DELTA.H.sub.atomiz):
The enthalpy of atomization is the energy involved in the transformation of 1 mole of a substance into its gaseous atoms, at the same temperature and pressure. Energy is always required to transform any substances, solid, liquid, or gas, into its gaseous atoms, so that the enthalpy of atomization, .DELTA.H.sub.atomiz, is always positive (heat is absorbed by the system).
Enthalpy of Dissociation (.DELTA.H.sub.diss):
This term is used to indicate the energy involved in the dissociation of a gaseous covalent molecule into its individual gaseous atoms at the same temperature and pressure as the original molecule. For these elements which exist at 25.degree. C. and 1 ATM as gaseous diatomic molecules, the enthalpy of dissociation is the same as the enthalpy of atomization given in published tables.
Enthalpy of Ionization (.DELTA.H.sub.ioniz):
We know that energy must always be supplied to remove an electron from a gaseous atom to form its monopositive gaseous ion, and that an even larger quantity of energy must be supplied to remove one or more additional electrons from the monopositive gaseous ion. Therefore, the enthalpy change .DELTA.H, for the removal of an electron either a neutral atom or a positive ion, always has a positive value.
The .DELTA.H of ionization of a gaseous atom to form a gaseous positive ion is called the ionization energy. Conversely, the enthalpy change involved in the process by which a gaseous atom takes on an electron to form a gaseous mononegative ion, is simply the electron affinity of the element.
Usually, the values of the ionization energies and electron affinities of elements are given for the process at 25.degree. C. and 1 ATM even though such reactions do not occur in actual practice under these conditions.
These terms indicate the energy involved in transforming 1 mole of a solid to a liquid, of a liquid to its vapour, and of a solid to its vapour, respectively. Again, both the reactant and the product of the transformation are at the same temperature and pressure.
Energy is always required to transform a solid to its liquid or vapour, and a liquid to its vapour, at the same temperature. Consequently, .DELTA.H.sub.fus, .DELTA.H.sub.subl, and .DELTA.H.sub.vap are always positive.
If a certain quantity of heat energy is required to melt 1 mole of a solid, or to evaporate 1 mole of a liquid, this same quantity of heat will be liberated when the liquid solidifies or the gas liquefies. Therefore, the heat of solidification of a substance is equal to its heat of fusion, but with a minus sign, and similarly the heat of liquefaction of a gas is equal to its heat of vapourisation, but with a minus sign.
Standard Enthalpy of Formation (.DELTA.H.sub.form):
The standard enthalpy of formation of a compound is defined as the heat involved in the reaction by which 1 mole of the compound is formed its elements, each element initially in its standard state, and at the same temperature as the compound formed. All elements in their standard states are conventionally as signed a heat of formation equal to zero. The standard enthalpies of formation, .DELTA.H.degree..sub.form, of compounds are usually given at 25.degree. C., therefore, we will follow this covention.
Normally, the .DELTA.H.degree. .sub.form of a compound does not have the capacity to decompose into its elements, or is stable toward decomposition into its elements on the other hand, if .DELTA.H.degree. .sub.form has an appreciably large positive value, the compound tends to decompose spontaneously into its elements at room temperature.
Variation of Enthalpy with Temperature:
The enthalpy, H of a system under constant pressure always increases with the temperature as shown by the following considerations. For any system which under goes a transformation at constant pressure and involves only pressure--volume work, we have the relationship: .DELTA.E=.DELTA.H-P.DELTA.V.
The internal energy of a system is directly proportional to its temperature, so that an increase in temperature means that .DELTA.E is positive.
Since most systems expand when the temperature increases P.DELTA.V is positive, we can conclude that an increase in temperature means an increase in the enthalpy of the system (.DELTA.H is positive) that is, H.sub.f is greater than H.sub.i.
The molar heat capacity of a substance at constant pressure, C.sub.p, is that temperature of 1 mole of any specified substance. Therefore, for a system consisting of a pure substance, the value of .DELTA.H is related to the heat capacity C.sub.p by the expression, .DELTA.H=C.sub.p .times..DELTA.T where .DELTA.T=T.sub.f -T.sub.i. In this expression C.sub.p is assumed to have a constant value within the considered range of temperature, T.sub.i -T.sub.f. Actually, the heat capacity at constant pressure C.sub.p is almost independant of temperature for solid and liquid substances, providing the range of temperature is not very wide. For gases it will vary appreciably. The value of C.sub.p is well known for many substances.
For all substances, the increase in enthalpy with increasing temperature follows a pattern similar to H.sub.2 O, although of course the values of the relative enthalpies of different substances at the same temperature may vary appreciably, especially since the melting and boiling points may differ by as much as 1,000 degrees or more for different substances.
Entropy of a System:
There exists a property called entropy S, which for systems in equilibrium states is an intrinsic property of the system, functionally related to the measurable co-ordinates which characterise the system. For reversible processes changes in this property may be calculated by the following equation ##EQU1## where T is the absolute temperature of the system.
The entropy change of any system and its surrounding, considered together, resulting from any real process is positive and approaches a limiting value of zero for any process that approaches reversibility. (Second Law of Thermodynamics).
In the same way that the first Law of Thermodynamics cannot be formulated without the prior recognition of internal energy as a property, so also the second law can have no complete and quantitative expression without a prior assertion of the existence of entropy as a property.
The second law requires that the entropy of an isolated system must increase, or in the limit, where the system has reached an equilibrium state, remain constant. For a closed (but not isolated) system it requires that any entropy decrease in either the system or its surrounding be more than compensated by an entropy increase in the other part, or in the limit, where the process is reversible, that the total entropy of the system plus surroundings remain constant.
The fundamental thermodynamic properties that arise in connection with the first and second laws of thermodynamics are internal energy and entropy. These properties together with the two laws for which they are essential, apply to all types of systems. However, different types of systems are characterised by different sets of measurable co-ordinates or variables.
Since we know that any property of a system whose change during a reaction depends only on the initial and final states is a thermodynamic function, we can now define its entropy. The entropy and its change in any reaction at constant temperature T is: ##EQU2## Since T always has a positive value, .DELTA.S is positive (S the entropy of the system increases) when the system absorbs heat from the surroundings (Q is positive) and vice-versa, the entropy S of the system decreases (.DELTA.S is negative) when the system gives up heat to the surroundings (Q is negative). It follows from the above definition that the entropy S of a system is expressed in units of energy divided by degrees of absolute temperature.
Since thermodynamic calculations usually use the mole as the basis for quantity of substance, .DELTA.S is expressed in units of CAL/MOL.times.DEG. K.
This unit is called the entropy unit (E.U.) to put our symbolic expression as ##EQU3## into words, for any transformation at constant temperature, the entropy change, .DELTA.S is equal to the heat exchanged by the system with the surroundings under reversible conditions, Q.sub.rev, divided by the absolute temperature, T at which the heat is exchanged. It follows, that for the same value of Q.sub.rev the value .DELTA.S is inversely proportional to the absolute temperature T.
When summarised, we can say: Entropy is a thermodynamic function which is a measure of the disorder of a system--a disorder may be thought of as having a two-fold character, a positional disorder, which is a disorder of molecular arrangement, and an energetic disorder which is a disorder of energy distribution related to the possible distributions of the energies among all the molecules of the system.
As the temperature becomes lower, the entropy of any system decreases, since both the positional and the energetic disorder decrease, in fact, at the lowest possible temperature, absolute zero, the entropy of any substance in the form of a perfect crystal, is taken to be zero.
Free Energy of a System:
The object of a thermodynamic analysis of a real process is the determination of the efficiency of the process from the standpoint of energy utilization. If W.sub.usefulmax is negative (work done by the system) the transformation can take place spontaneously, if W.sub.usefulmax is positive (work is done on the system) the transformation is not spontaneous, and in fact, work must be expended to carry it out. EQU W.sub.usefulmax =.DELTA.H-Q.sub.rev and EQU Q.sub.rev =T.DELTA.S and EQU W.sub.usefulmax =.DELTA.H-T.DELTA.S
Again, W.sub.usefulmax represents the change of the system to do useful work in passing at constant temperature and pressure from a specified initial to a specified final state. This capacity of the system to do useful work when it is in a given state is called the Gibbs Free Energy, and is indicated as G. Therefore, EQU W.sub.usefulmax =G.sub.f -G.sub.i =.DELTA.G EQU .DELTA.G=.DELTA.H-T.DELTA.S
These expressions tell us that when a system passes at constant temperature and pressure from a specified initial state to a specified final state, the change in the free energy of the system, EQU .DELTA.G=G.sub.f -G.sub.i
is equal to the maximum useful work involved in the transformation. Therefore, if W.sub.usefulmax is positive (work done on the system), the free energy of the system increases in the transformation (.DELTA.G is positive). Conversely, if W.sub.usefulmax is negative (work done by the system), the free energy of the system decreases (.DELTA.G is negative).
Also, at constant temperature and pressure, the free energy change, .DELTA.G is equal to the change in the enthalpy, .DELTA.H minus the product of the absolute temperature T and the entropy change .DELTA.S.
When we consider the unit involved in the equation, .DELTA.G=.DELTA.H-T.DELTA.S, then if we express .DELTA.H in CAL/MOLE, T in DEG.K. and .DELTA.S in CAL/MOLE.times.DEG. then .DELTA.G is expressed in CAL/MOLE. Therefore, .DELTA.G represents the quantity of energy per mole, which is free to do useful work, when a system passes from a specified initial to a specified final state.
The .DELTA.G values for some specific processes are often designated by placing the abbreviated name of the process as a subscript after the symbol .DELTA.G, just as we did for the .DELTA.H values. For example, we write .DELTA.G.sub.fus, .DELTA.G.sub.vap, and .DELTA.G.sub.diss to indicate respectively, the free energy change of a fusion, a vaporization process, and the dissociation of a molecular substance into its isolated atoms.
Standard Free Energy Formation (.DELTA.G.degree. .sub.form) of a Compound:
This term is used to indicate the free energy change of the reaction in which a specified compound, at a certain temperature and pressure, is formed from its elements in their standard state at the same temperature by convention, all elements in their standard state at that time are assigned a .DELTA.G.degree. .sub.form equal to zero.
Influence of Temperature on the .DELTA.G of a Reaction.
The value of the free energy change, .DELTA.G for a specified reaction, therefore, the capability of the reaction to take place spontaneously depends largely on the temperature at which the reaction occurs. In other words, a reaction which is thermodynamically forbidden, (.DELTA.G=positive) at a certain temperature and pressure, can at a higher temperature be thermodynamically permitted, (.DELTA.G=Negative). Actually, the change from a thermodynamically forbidden to a permitted reaction is predominantly the result of the effect of the absolute temperature, T on the magnitude of the T.DELTA.S energy term.
The following, generally holds true for any reaction in which the entropy of the system increases (disordering reaction).
If the reaction cannot occur spontaneously at a given temperature because of a large unfavourable .DELTA.H (strongly endothermic reaction), a rise in temperature will make the favourable T.DELTA.S factor relatively more important.
Consequently, there is always a certain temperature, even though it is sometimes an extremely high one, at which the favourable T.DELTA.S term finally out weighs the unfavourable .DELTA.H term, so that the reaction becomes thermodynamically permitted. One kind of endothermic reaction which can always be obtained spontaneously provided the temperature is high enough, is the dissociation of substances (elements and compounds) into their atoms.
In the process of atomization of a substance the products always have a greater entropy than the reactant, and the process always results in an increase in entropy (.DELTA.S is positive ).
Of course a large quantity of energy is required, for an atomization reaction, .DELTA.H.sub.atomiz has a large positive value, because atomization involves the breaking of the strong chemical bonds which hold together the atoms of the reactant in the molecule, or in the ionic crystal, or in the metallic state.
In terms of practical utility related to the present invention, or the preferred catalysts or reactants used in the present invention, will follow the basic Laws of Science stated above. It has been previously stated in the content of the objects of the present invention, that it will use more than one catalyst, one of which will be employed as a diffusion membrane as well as a reactant. The preferred material used in the present invention to provide the multiple function required, is palladium or alloys of palladium.
The basic Laws of Science related to palladium or its alloys as a reactant has been previously stated, however, the rate of diffusion of hydrogen through the actual solid membrane need to be defined.
The rate of diffusion of hydrogen through the solid membrane will be governed by the diffusion coefficient, D, appropriate to its chemical composition (i.e., in the case of palladium whether this is either .alpha.- or .beta.- phase), and to the concentration gradient across the membrane and to its thickness.
In addition, however, the overall rate of permeation also is dependent on the catalytic efficiency of the intermediate reaction steps on the surfaces of exit and entry, and also on the transport of the products (superheated steam, hydrogen/oxygen) up to and away from these surfaces.
Next it seems worthwhile to briefly introduce the simplest diffusion relationships which might be expected to apply when diffusion through the hydrided palladium or its alloys is indeed the slowest stage.
Provided transport of hydrogen through the solid palladium governs the rate of permeation, and further provided that there are no sharp discontinuities of composition in the solid (such as boundary between regions of .alpha.- and .beta.- phases) the rate of permeation, P can be written in terms of Fick's First Law as: ##EQU4## where D(CM.sup.2 SEC.sup.-1) is the diffusion coefficient for the particular phase, .alpha.is the thickness of the membrane and C.sub.entry and C.sub.exit are the concentrations of hydrogen dissolved in the membrane at the interfaces of entry and exit, respectively.
Then with .alpha.expressed in CM. and the concentrations of hydrogen in the solid expressed as relative volumes (i.e. ccH.sub.2 /ccPd-cf Table 2.1) the permeation rates are obtained as ccH.sub.2 /cm.sup.2 /cm.
Expressing Fick's Equation in Terms of Pressure:
Relationships between permeation rates and diffusion constants often have been written in terms of gas pressures or powers of the gas pressure rather than in terms of concentrations. This is a more convenient form in which to obtain a permeation rate by insertion of experimentally determined values of pressure and perhaps it may also be the result of widespread interest in the interpretation of problems concerning the relationships between diffusion rates and pressures at very low pressures. However, from the view point of those not well versed in diffusion problems it rather directs attention away from the fact that the critical variable is the solubility of gas in the solid as a function of pressure.
In practice, experimental results show that over wide ranges of pressure the solubility of hydrogen in nickel or platinum is approximately directly proportional to the square root of pressure i.e., EQU C=kp.sup.1/2
A similar approximation, with characteristic values of K, is found to hold fairly well for the relationship between pressure and the solubility of hydrogen in the .alpha.-phase of the Pd/H system.
Substitution for C in equation ##EQU5##
Better approximations of the correct concentration have been achieved by equating concentration (hydrogen content) to alternative powers of pressure, leading to a more generalised relationship ##EQU6## where values of .eta.=0.8 or 0.68 at particular temperatures and ranges of pressure. In general, values of .eta. are obtained from plots of LogP against Logp.
In deriving diffusion coefficients from permeation rates by means of the Fick First Law Equation, it has to be assumed that equilibration between gas and solid is complete and rapid at both the entry and exit surfaces, so that it is diffusion through the solid which is the slow and rate-determining step, and so that the concentrations of hydrogen at the surfaces may be derived from the gas pressures through p-C-T relationships. In general, the surfaces of palladium membranes do not readily equilibrate with molecular hydrogen at room temperature. However, success in this respect has been achieved by various activation procedures, such as by plating on a layer of palladium black, or by contacting the surfaces with material such as copper which are more efficient than palladium for dissociating molecular hydrogen and from which the dissociated atoms migrate to the palladium surface. The preferred material used in the present invention for activation is copper.
At present, it seems generally accepted that temperatures exceeding 250.degree. C. is needed to achieve the required permeation rates. The experimental work has shown the necessity maintaining temperature above 250.degree. C. as well as movement and circulation of the gas at the input or upstream side of the membrane. Without such circulation, permeation rates have been found to decrease to negligibly low values. When this movement or circulation maintained an excellent results have been obtained and the permeation rates were maintained for prolonged periods.